Blown-film apparatus is used to manufacture plastic sheets, bags and the like. A molten tube of plastic is extruded from an annular die and then stretched and expanded to a larger diameter and a reduced thickness by the action of overhead nip rolls and internal air pressure.
To control the circumference of the finished tube, it is generally necessary to adjust the volume of air captured and held inside the tube between the annular die and the overhead nip rolls. It has been conventional to adjust the entrapped volume of air by operating valves in the small inflation line connected to the tube interior, i.e. to add or remove small amounts of the entrapped air. These valves were opened and closed in response to measurements of tube size.
Some of these inflation systems measured tube size using optical beams tangent to the tube and incident upon photocells, e.g. Uemura et al., U.S. Pat. No. 3,932,080; Suh et al., U.S. Pat. No. 3,159,698; Tsuboshima et al., U.S. Pat. No. 3,499,064; and Matsuo et al., U.S. Pat. No. 3,400,184.
Mechanical feelers have also been used to control the entrapped volume, e.g. Hearns et al., U.S. Pat. No. 3,700,370.
To increase production speeds of blown film lines, a continuous stream of cooling air has been admitted into the tube through passages in the die, directed against the inner wall of the tube and removed through passages in the die. In such dies, in order to regulate the flow rate in a dynamic manner a mechanical feeler that follows the wall of the tube may be used. Schott, U.S. Pat. No. 3,980,418, shows a single feeler mechanically connected to a pneumatic regulator valve which, through a pneumatic cylinder, proportionally controls a flapper valve in the internal air supply line. As with all mechanical feelers, however, deformations are left in the film and a degree of inaccuracy must exist due to the need of the film to apply pressure to the feeler in order to produce a response. Furthermore, with such feelers, tube size measurements must be made beyond the molten region of the tube to avoid serious deformations in the tube wall as the result of contact and locallized cooling by the feelers. Making the measurement away from the molten region can introduce detrimental delay into the control system, and reduce accuracy.
In another system for controlling tube size in a film line employing a continuous stream of cooling air (shown in the assignee's copending application of Schott, Ser. No. 250,617, filed Apr. 3, 1981 (C.I.P. of Ser. No. 964,983, abandoned filed Nov. 30, 1978), entitled "Control of Tubular Film Size"), surface deformations are avoided by use of a tangential energy beam system to sense tube diameter in the molten region within a tube diameter range of 1.25 inches, without repositioning the sensors. Air flow into the tube is regulated by a control circuit operating a motorized valve in the cooling air duct, as described in Mulcahy, U.S. Pat. No. 4,243,363.
It is an objective of this invention to provide improved gauging and controlling apparatus for blown film extruder systems employing a continuous stream of cooling air that monitors tube diameter over a wide range of values without repositioning. It is a further objective to provide an apparatus capable of determining the tube circumference close to the frost line, without deforming or locally cooling the tube, for prompt and accurate control of internal cooling air flow.